Honestly, the kidnapping and murder of Otto Warmbier is causus belli enough. Whether it would be a good idea is up in the air, but justified? Absolutely.

I thought he was arrested while he was in North Korea for committing minor vandalism?

North Korea's story has been exposed as a lie. They claimed he was a member of a Church group conspiring to convert people, and his supposedly stealing the banner was part of that "mission". That story is unlikely for patently obvious reasons.

Their "evidence" is a filmed video of him supposedly taking down the banner, which was likely staged once they had decided to kidnap him and hold him for ransom. Why they decided to murder him, we'll never know.

Honestly, the kidnapping and murder of Otto Warmbier is causus belli enough. Whether it would be a good idea is up in the air, but justified? Absolutely.

So we should send millions to their death, and allow tens of millions of civilians to be killed in their home, because one person died who shouldn't have been there to start with? Brilliant logic, glad you aren't in charge.

Millions of people aren't going to be sent to die. The goal of assassinating this psychopathic tyrant would be to prevent such a war.